Document error cases and limits in README

[iexwr]

Context

The contract README documents functions but not the failure modes contributors must respect when calling or extending the contract.

Acceptance criteria

• Add an "Error handling" subsection to README.md listing every panic!/assert! condition and what triggers it.
• Document the 128-byte memo cap and the amount > 0 / expires_in > 0 rules.
• No code changes required.

Technical notes

• Cross-check the list against src/request.rs so it stays accurate.

Testing

• make test unchanged; docs only.